High Dose Rate (HDR) brachytherapy is a form of radiotherapy for cancer treatment that can deliver a high dose of radiation to the target volume, while minimising dose to the nearby healthy tissue. HDR brachytherapy is delivered by one or more high activity sources, moved through catheters placed inside the tumour or close to the treatment volume, dwelling at set positions for specified times.
HDR brachytherapy treatment can be complex, usually requiring computed tomography (CT) data to accurately plan the treatment on a specialised treatment planning system (TPS). The treatment prescription dose can be delivered in single or multiple treatment fractions. Source dwell positions and dwell times are calculated by the TPS to achieve the prescribed dose distribution around the target volume. A remote afterloader unit drives the radioactive source to dwell positions in the implanted catheters, specified by the treatment plan.
Accurate treatment delivery relies on the source dwell positions occurring in the correct locations relative to the surrounding anatomy and according to the approved treatment plan. Inaccuracies in treatment delivery can occur due to several reasons; implanted catheters move inside the patient after CT imaging1, specified length of the catheter is incorrect, catheter tip offset is incorrect, transfer tube connection is incorrect (afterloader to patient catheter), wrong treatment plan is delivered, remote afterloader malfunctions, source calibration data in the brachytherapy treatment console is incorrect2.
HDR brachytherapy for prostate cancer, either in combination with external beam radiation therapy (EBRT) or as “monotherapy”, has theoretical radiobiological and physics advantages, as well as economic advantages, over EBRT alone for achieving local control for prostate cancer. It is known that “dose-escalation” in the treatment of prostate cancer with radiation improves disease control3,4. Progress in escalating dose in HDR treatment has been hampered in comparison to dose-escalation with EBRT—despite putative theoretical advantages—because techniques for real-time individualized treatment verification have been developed for EBRT side by side with the research effort on dose-escalation, thereby enabling certainty about patient safety. Similar standards of verification for HDR brachytherapy would enable economic, radiobiological and physics related advantages of HDR brachytherapy.
Current strategies for HDR treatment verification usually consist of detectors to measure the dose at a single point within the treatment volume. Thermoluminescent dosimeters (TLDs)5, metal oxide semiconductor field effect transistors6 (MOSFETs), diodes7, diamond detectors8 and scintillation detectors9,10 are devices that can be used for in vivo dosimetry for brachytherapy. To overcome the limitation of single point detectors, some authors have used sets of detectors to measure a simple dose profile of a few points within the treatment volume11,12. A limitation of this method is that the one dimensional profile produced has a low spatial resolution and cannot fully verify the dose distribution delivered to the entire target volume. An additional weakness to point dose methods is the difficulty of localising the detector in relation to the target volume, making dose comparison with the TPS difficult and questionable. Point dose profiles measured inside critical organs often require data profile shifts in order to match measured data with TPS profiles13. Detectors have to be placed against or inside the organs, making routine data collection difficult, and causing discomfort to the patient. Establishing a routine in vivo dosimetry protocol for HDR brachytherapy using point detectors is problematic and unlikely to achieve widespread acceptance.